Leuco dyes are known in the prior art to exhibit a change from a colorless or slightly colored state to a colored state upon exposure to specific chemical or physical triggers. The change in coloration that occurs is typically visually perceptible to the human eye. All existing compounds have some absorbance in the visible light region (400-700 nm), and thus more or less have some color. In this invention, a dye is considered as a “leuco dye” if it did not render a significant color at its application concentration and conditions, but renders a significant color in its triggered form. The color change upon triggering stems from the change of the molar attenuation coefficient (also known as molar extinction coefficient, molar absorption coefficient, and/or molar absorptivity in some literatures) of the leuco dye molecule in the 400-700 nm range, preferably in the 500-650 nm range, and most preferably in the 530-620 nm range. The increase of the molar attenuation coefficient of a leuco dye before and after the triggering should be bigger than 50%, more preferably bigger than 200%, and most preferable bigger than 500%. In some cases the leuco compound can be used with other colorants, and other colorants may interfere the absorbance of the leuco dye before and/or after triggering. In this case, it is best to measure leuco dye and its colored form separately in a solvent. The method of measuring molar attenuation coefficient of a compound is well-known. A description of the molar attenuation coefficient (in the name of molar absorption coefficient) and its measurement method can be found in Vogel's Textbook of quantitative chemical analysis, 5th edition, 1989, John Wiley and Sons. The chemical or physical triggers that bring about the coloration change include, but are not limited to, oxidation, intramolecular ring opening, pH change, and exposure to heat and/or cold or light (e.g. UV light). For example, triphenylmethane (“TPM”) compounds, one class of leuco dyes, are useful in applications such as photoimaging and typewritten ribbons whereby microencapsulated TPMs are brought into contact with an acid source and images are generated when pressure or heat is applied. These dyes are described, for example, in Chemistry and Applications of Leuco Dyes (edited by Ramaiah Muthyala, pp. xi-xiii; 151-152).
The use of polymeric colorants for coloring consumer products is well known in the prior art.
As one non-limiting example, the use of whitening agents, either optical brighteners or blueing agents, in textile applications is known. As textile substrates age, their color tends to fade or yellow due to exposure to light, air, soil, and natural degradation of the fibers that comprise the substrates. Thus, the purpose of whitening agents is generally to visually brighten these textile substrates and counteract the fading and yellowing of the substrates.
Previous attempts to add bluing agents to fabric care products have used preformed pigments or dyes such as azo dyes, triaminotriphenyl methane compounds, triphenyl methane compounds and anthraquinone colorants. U.S. Pat. No. 4,137,243 to Farmer teaches polymeric anthraquinone-derived colorants which exhibit improved light and alkali fastness properties. Farmer also discloses that these colorants may be incorporated into detergent compositions to provide coloration or blueing effect for the detergent composition. These types of colorants must therefore be alkali fast, in order to withstand the alkaline conditions of the detergent composition. The colorants should also be water fugitive so as to not stain the textile articles washed with the colored detergent composition. However, Farmer does not disclose leuco colorants that have the ability to transform from a colorless to a colored state upon exposure to certain physical or chemical changes.
U.S. Pat. No. 5,039,782 to Langer et al. discloses a copolymer whitening agent that contains a fluorescent group and a hydrophilic group. The whitening agent is preferably 4,4′-bis(carbomethoxystilbene), and the hydrophilic group is preferably a mixture of polyethylene glycol and ethylene glycol. The copolymer optionally contains a hydrophobic monomer portion, such as polyethylene terephthalate, in order to better adhere the polymer to a hydrophobic surface (like polyester fabric or soiled cotton fabric). The resulting copolymer provides dual functionality as a whitening agent and for providing soil release to fabrics. However, it is apparent from the test data provided in Table 3 of the reference that the copolymer fails to provide adequate whitening for soiled cotton fabrics without the addition of a second whitening agent (i.e., Tinopal). Furthermore, Langer et al. fail to disclose leuco colorants that exhibit a reversible transformation from a colorless to a colored state.
U.S. Pat. No. 7,208,459 to Sadlowski et al. discloses the use of hueing dyes in laundry detergent compositions for combating the yellowing of fabrics. The hueing dye is designed to avoid significant build up of the dye on fabric so that the fabric does not exhibit a bluish tint, for example, after repeated exposure to the hueing dye present in laundry detergent. The laundry detergent composition is comprised of a surfactant and a hueing dye. The surfactant may be anionic, nonionic, cationic, zwitterionic, and/or amphoteric in nature. The hueing dye is characterized by having a hueing efficiency of at least 10 and a wash removal value in the range of between 30% and 80%. Exemplary dyes which exhibit these properties include certain categories of dyes that contain blue or violet chromophores, such as triarylmethane dyes, basic dyes, anthraquinone dyes, and azo dyes. However, this reference fails to disclose the use of unsubstituted or leuco colorants as described by the present invention.
Thus, it is contemplated to be within the scope of the present invention that the leuco colorants described herein may be ideally suited for use as whitening agents. Many of the whitening agents that are commercially available exhibit a dark color, e.g. a dark blue color, when added to a laundry care composition, such as a laundry detergent, rinse aid, fabric softener, and the like. For instance, the triphenyl methane and thiazolium structures are positively charged colored species. With colored species such as these, the amount of color is visually apparent and may be an undesired shade for consumers. Powdered detergent systems typically use colored speckles to reduce apparent color of the detergent by incorporation of color within the interior of a speckle or granule. Liquid products often incorporate opacity modifiers to reduce the apparent darkness of the product.
The need exists for an effective whitening agent that consumers can use without concern that the garments and other textile substrates will be irreversible stained with the laundry detergent composition that contains a whitening agent. Thus, the colorless leuco colorants described herein may be added to laundry care compositions without fear of staining, since these colorants are colorless when added to the laundry machine and only exhibit color during the laundry cycle and/or upon exposure to ultraviolet light when the whitening effect is achieved.
The present invention offers advantages over U.S. Pat. Nos. 4,137,243 and 5,039,782 and US Patent Application Publication No. 2005/0288206 as this invention takes advantage of colorless compounds that can be converted to colored compounds with the addition of certain physical and/or chemical catalysts. Such compounds are useful for many consumer products, including, but not limited to, their use as whitening agents in laundry care compositions. As whitening agents, the colored compounds exhibit the desired wavelengths in the range of blue, red, violet, purple, or combinations thereof upon exposure to ultraviolet light (or, they absorb light to produce the same shades) in order to neutralize the yellowness of textile substrates and provide a brightening effect.